Ultra-broadband enhanced absorption of metal surfaces structured by femtosecond laser pulses.

We investigate the enhanced absorption properties in a wavelength range of 0.2-25 microm for NiTi alloy targets structured by femtosecond laser pulses in air. Three different types of surface structures are produced with varying laser fluences. Measured reflectances through integrating sphere technique show that their couplings of incident electromagnetic irradiations are improved greatly over the broadband wavelength range. In particular, for coral-like micro-structures on the metal surfaces, approximate 90% absorption can be achieved from ultraviolet to mid-infrared region. Cut-off wavelengths of the enhanced absorption for the varied dimensional surface structures are determined experimentally. Chemical analysis by X-ray photoelectron spectroscopy indicates that blackness of metal surfaces is not attributed to the change in elemental composition. The physics of such remarkable absorption for the structured metal surfaces are discussed as well.